Fastening system (1) for fastening an attachment (18) to a substrate (13) with an insulation layer (16), the fastening system (1) comprising an anchor bolt (2), with a first area (3) for anchoring in the substrate (13), a second area (4) to which the attachment (18) can be fastened, and a stop (5), a mounting tool (6) to form an opening in the insulation layer (16), and a sealing element (7) adapted to be disposed at the stop (5) after the mounting tool (6) has been removed and to seal the opening in the insulation layer (16) as well as a corresponding method.